Sometimes the fiber optic installer is faced with a fiber optic cable or cable plant that, at first glance, doesn’t seem reasonable, necessary or even possible to test. However, most situations can be handled with regular test equipment plus some adapter cables and a thorough understanding of standard procedures. Here are some examples from recent jobs that have confused installers.
The first example is a premises cabling network that used a factory-made prefabricated cabling system; it was shipped to the customer location for installation. The system was a ribbon cable backbone with each 12-fiber ribbon in the backbone cable terminated in a mechanical transfer pull-off (MTP) connector. At either end, the cable plant was converted into LC connectors using special MTP-to-12 LC connector breakout cables.
The contract with the customer called for end-to-end testing per TIA 568-C standards. The contractor had already tested insertion loss on most of the cable plant using a light source and power meter, when the customer questioned his methodology. The contractor was using a two-cable reference (OFSTP-14, Method A) since his testing equipment had SC connectors and needed adaptation to the LC connector for testing. The customer had assumed that TIA 568-C still required using an OFSTP-14, Method B, one-cable reference, but the latest TIA 568-C revision recognizes that the new types of connectors, such as the LC and the multifiber/ribbon MTP, require different test methods; it allows those to be used. Thus, the contractor’s test methods were in compliance with TIA 568-C as required by the contract.
Although every fiber tested showed low loss, the customer also wanted the MTP connectors tested. The contractor argued that his end-to-end loss tests showed the losses of the MTP connectors were within specifications and that any further testing would not be productive. But the customer, probably prompted by a vendor, wanted further testing using an optical time-domain reflectometer (OTDR), which is now called Tier 2 testing in TIA 568-C.
The contractor, who really did not want to tie up his crew for another week, argued (correctly, in this case) that the cables were too short for OTDR testing to show anything. However, the customer was adamant and, more importantly, willing to pay for the contractor’s extra time, so he got his mostly worthless OTDR test results.
The contractor could have tested the MTP connectors with his light source and power meter if he had some matching SC or LC-to-MTP reference cables, but he would have had to use a three-cable reference, which is the only way to test these multifiber connectors. However, the measurement’s uncertainty would have been very high. The proper method is to test the end-to-end loss of the installed cable plant with the MTP connector losses included in the results.
Another instance involved testing plastic-optical fiber (POF) used on the controller of an industrial robot. POF is often used in industrial applications, and some customers have decided to make their own cables rather than buy them at considerably higher costs. The technician working with these cables wanted to know how to test them. POF connectors are designed to connect directly to sources or detectors, not to mate two fibers. Since interconnecting hardware is not available, you cannot perform a standard cable test, which requires connecting the cable under test to a reference cable. Furthermore, POF generally operates at a different wavelength with 650 nm red light-emitting diodes (LEDs).
To test POF, you need a special meter with an adapter for the POF connector and either a special 650 nm LED source or a system transmitter you can use. You also need one known good cable, which will be your reference for all tests. Testing is as simple as connecting the good cable between a source and the meter and checking the power level. Then, you must replace the reference cable with an unknown cable and measure the power. If the power is higher, the cable is better than the reference cable; lower power means it’s worse. Reverse the cable to ensure both connections are good. Knowing how bad a cable can be while still functioning in a system means knowing which cables are good or bad.
The biggest problem contractors seem to have is with OTDR testing. Customers are sometimes convinced that only an OTDR test can validate an installation, even though such testing is often inappropriate. In the next few months, this column will explore OTDRs and help you to understand their proper use.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.